I notice this, as well. I have a million things to say on this topic. Here are my thoughts.
First of all, I happen to be a healthy weight for my height, but I wasn't always.
Food can be a source of immense comfort to a lonely individual, and when you have trouble socializing then food becomes a very big, stimulating part of your day. When you have Netflix and a jar of Nutella then who the heck needs friends anyway, right?
I used to binge eat without even realize that I was binge eating. I kind of ignored remarks from family members who would try to talk to me about my "weight problem." Little did they know it was more of an eating problem, but of course all they saw was my weight. I'd get lots of, "you have such a pretty face" and "you'd look so good if you lost weight" and blah blah blah. I was probably thirty pounds overweight when I finally decided to go on a diet.
I don't feel like going into details, but to make a long story short, I took it too far, lost too much weight, developed an eating disorder, and when I tried to eat normally again, I switched right back to binge eating because food comforted me, and I had no friends. So weight comes back, I discover bulimia, and weight kinda comes off again, rinse, recycle, repeat. This has literally gone on with me for 13 years. I consider myself to be in recovery from a full blown eating disorder, and I may not eat normally now (I eat a big meal once at night and sometimes a snack or two after...this just works for me, I'm not suggesting this to anyone) but I am definitely more in control of myself with food.
I feel like it's easy to become overweight when food is a source of comfort, and we autistic fools love to be comforted!
I feel like people with autism tend to be very picky about food, and it can lead to cutting out a lot of food from your diet, which is why many people with autism are underweight.
Autism most definitely does not have anything to do with metabolism, but metabolism has to do with more than our bodies; it is connected to our minds and our spirits as well. (I'm obsessed with metaphysics, okay?) I think the key to being a healthy weight is to treat food as if it is sacred, and enjoy it fully instead of eating mindlessly in front of the computer or TV (although I am so guilty of doing this, and I usually eat more than my body needs when I do this, and I just gotta deal with the consequences...that's what exercise is for, right?) Food is more than just fuel - it's a gift to us, and we should cherish it.
Junk food may not be good for us, and it can and totally will mess with our moods, but if you enjoy the heck out of it, then allow yourself to eat the gosh darn cheetos! Don't deprive yourself! LIVE!
Thanks for bringing this topic up. It's really interesting.